1. Field of the Invention
The present invention relates to a video-signal-processing device, an imaging apparatus using the same, a method for processing a video signal, and a program product therefor.
2. Description of Related Art
When taking an image, an imaging apparatus has performed any Knee correction to compress highlight areas of an imaging signal output from an image pick-up device into its dynamic range (saturation signal quantity) and outputs it as a video signal having such a dynamic range as to comply with the broadcast standards.
According to this Knee correction, if an input signal Ein is less than a Knee point level KP, an uncompressed output signal Eout can be obtained as expressed in a following Equation (1); and if the input signal Ein is of the Knee point level KP or higher, as expressed in a following Equation (2), the input signal Ein is subject to compression processing by using a compression ratio (Knee slope) KS, thereby generating the output signal Eout.Eout=Ein(Ein<KP)  (1)Eout=KP+KS(Ein−KP)(Ein≧KP)  (2)
It is noted that FIG. 1 shows characteristics of the Knee saturation.
If performing such the Knee correction on each of the three primary-color signals Rin, Gin, and Bin, as shown in FIG. 2, a Knee correction section 10 is equipped with a level conversion section as well as clip sections 12r, 12g, and 12b; with this, the level conversion section 11 compresses the three primary-color signals Rin, Gin, and Bin to generate three primary-color signals Rkn, Gkn, and Bkn, and then, the clip sections 12r, 12g, and 12b respectively clip the three primary-color signals Rkn, Gkn, and Bkn by a white clip level WC to have their levels fall within a level range that conforms to the broadcast standards.
FIG. 3 shows characteristics of a Knee correction section according to related art, indicating signal processing results obtained by performing the Knee correction and clip processing on each of the color signals. It is noted that in FIG. 3, for example, a red color signal Rout is indicated by a solid line, a green color signal Gout is indicated by a dotted line, and a blue color signal Bout is indicated by a dash-and-dot line (and so on).
With this, if a subject has a higher luminance and, for example, the red color signal Rin reaches the Knee point level KP or higher, only level compression of the color signal Rin is performed. Accordingly, if a proportion among the color signals Rout, Gout, and Bout is changed, a hue is changed. If the subject has an even higher luminance and, for example, the color signal Gin reaches the Knee point level KP or higher, level compression is performed on the color signals Rin and Gin. Accordingly, also when the color Gin reaches the Knee point level KP or higher, the hue is changed. Further, if the color signal Rin that has undergone level compression reaches the white clip level WC, clip processing is performed thereon to restrict the color signal Rout to the white clip level WC. Therefore, in this case also, the proportion among the color signals Rout, Gout, and Bout is changed, and the hue is also changed. Similarly, also when the color signal Gin that has undergone level compression reaches the white clip level WC, the color signal Gout is restricted to the white clip level WC, so that the hue is changed.
Japanese Patent Publication No. 3509448 discloses a video-signal-processing device in which even if any Knee correction or clip processing is performed on the three primary-color signals, an arrangement is carried out so that a hue of an image based on the post-Knee correction or post-clip processing of three primary-color signals may be kept constant.
If the hue of an image based on the post-Knee correction or the post-clip processing of three primary-color signals is kept constant by using the video-signal-processing device disclosed in the above Japanese Patent, a saturation decreases in a bright area thereof to fade their colors as compared with a case where the Knee correction or clip processing is performed thereon as shown in FIG. 3.
FIG. 4 shows a configuration of such the video-signal-processing device. The three primary-color signals Rin, Gin, and Bin are respectively supplied to a luminance signal generation circuit 21 and the respective multipliers 24r, 24g, and 24b in a luminance conversion section 20. The luminance signal generation circuit 21 receives the three primary-color signals Rin, Gin, and Bin to generate a luminance signal Yin and supplies it to a luminance Knee correction circuit 22 and a divider 23. The luminance Knee correction circuit 22 performs any Knee correction on the luminance signal Yin and supplies a post-Knee correction luminance signal Yyk to the divider 23, subtracters 32, 33, 35r, 35g, and 35b, and adders 37r, 37g, and 37b. 
The divider 23 divides the luminance signal Yyk by the luminance signal Yin to calculate a luminance adjustment compression ratio KY (=Yyk/Yin) and supplies it to the multipliers 24r, 24g, and 24b. 
The multipliers 24r, 24g, and 24b multiply the three primary-color signals Rin, Gin, and Bin with the luminance adjustment compression ratio KY to generate three primary-color signals Ryk, Gyk, and Byk, respectively.
In other words, the three primary-color signals Ryk, Gyk, and Byk are obtained as a result of computations of following Equations (3) through (5), respectively. Therefore, even if level compression is performed, the hue can be kept constant.Ryk=(Yyk/Yin)*Rin  (3)Gyk=(Yyk/Yin)*Gin  (4)Byk=(Yyk/Yin)*Bin  (5)
The three primary-color signals Ryk, Gyk, and Byk are respectively supplied to a maximum-value signal setting circuit 31 and the subtracters 35r, and 35g, and 35b in a saturation conversion section 30. The maximum-value signal setting circuit 31 selects a highest level one among the three primary-color signals Ryk, Gyk, and Byk and supplies this selected color signal as a maximum-value signal DMAX to the subtracter 32. The subtracter 32 subtracts the luminance signal Yyk from the maximum-value signal DMAX and supplies a subtraction result (DMAX−Yyk) to a divider 34.
The subtracter 33 subtracts the luminance signal Yyk from the white clip level WC and supplies a result of this subtraction (WC−Yyk) to the divider 34.
The divider 34 divides the subtraction result (WC−Yyk) by the subtraction result (DMAX−Yyk) to calculate a compression ratio KA as shown in following Equation (6) and supplies it to a compression ratio restriction circuit 38.KA=(WC−Yyk)/(DMAX−Yyk)  (6)
If the compression ratio KA is larger than “1”, the compression ratio restriction circuit 38 restricts the compression ratio KA to “1” and supplies it as a saturation compression ratio KC to the multipliers 36r, 36g and 36b. On the other hand, if the compression ratio KA is not larger than “1”, the compression ratio restriction circuit 38 supplies the compression ratio KA calculated by the divider 34 to the multipliers 36r, 36g and 36b as the saturation compression ratio KC.
The subtracter 35r subtracts the luminance signal Yyk from the color signal Ryk and supplies a result of this subtraction to the multiplier 36r. Similarly, the subtracter 35g subtracts the luminance signal Yyk from the color signal Gyk and supplies a result of this subtraction to the multiplier 36g and the subtracter 35b subtracts the luminance signal Yyk from the color signal Byk, and supplies a result of this subtraction to the multiplier 36b. 
The multiplier 36r multiplies the subtraction result obtained through the subtracter 35r by the saturation compression ratio KC and supplies a result of this multiplication to the adder 37r. The adder 37r adds the luminance signal Yyk to the multiplication result received from the multiplier 36r to generate a color signal Rcj and supplies it to a clip section 41r. 
Similarly, the multipliers 36g and 36b multiply the subtractions results obtained through the subtracters 35g and 35b by the saturation compression ratio KC and supply multiplication results to the adders 37g and 37b, respectively. The adders 37g and 37b add the luminance signal Yyk to the multiplication results received from the multipliers 36g and 36b to generate color signals Gcj and Bcj and supply them to clip sections 41g and 41b, respectively.
The clip sections 41r, 41g, and 41b are respectively supplied with the white clip level WC. These clip sections 41r, 41g, and 41b perform any clip processing on the three primary-color signals Rcj, Gcj, and Bcj received from the adders 37r, 37g, and 37b to generate clip-processed signals and output them as three primary-color signals Rout, Gout, and Bout, respectively.
In other words, the three primary-color signals Rcj, Gcj, and Bcj obtained through performing level conversion on the three primary-color signals Ryk, Gyk, and Byk at the saturation conversion section 30 indicate results of computations by following Equations (7) through (9).Rcj=Yyk+((WC−Yyk)/(DMAX−Yyk))(Ryk−Yyk)  (7)Gcj=Yyk+((WC−Yyk)/(DMAX−Yyk))(Gyk−Yyk)  (8)Bcj=Yyk+((WC−Yyk)/(DMAX−Yyk))(Byk−Yyk)  (9)
By thus performing level compression at the luminance conversion section 20 and performing level conversion at the saturation conversion section 30, as shown in FIG. 5A, if then a brightness becomes higher than a level La, a luminance signal, not shown, generated by using the three primary-color signals Rin, Gin, and Bin reaches a Knee point level LP or higher, so that level compression is performed on the three primary-color signals Rin, Gin, and Bin with a hue thereof being kept constant, to generate the three primary-color signals Ryk, Gyk, and Byk. Then, if any one of the three primary-color signals Ryk, Gyk, and Byk reaches the white clip level WC, any level conversion is performed on saturation components of these signals using the saturation compression ratio KC, thereby adjusting other primary-color signals so that the hue may be constant to generate the three primary-color signals Rout, Gout, and Bout.
Thus, keeping the hue constant allows, in a case where, for example, a luminance level when imaging a person is high, the image of this person to be prevented from being displayed yellowish as if the person is unhealthy. Further, increasing the saturation compression ratio KC enables a gradation of a bright area to be made more visible.